:root {
  --primary-color: #007aff;
  --bg-color: #f2f2f7;
  --text-color: #000;
  --board-bg: #fff;
  --border-light: #ccc;
  --border-bold: #333;
}

body {
  margin: 0;
  padding: 0;
  font-family: -apple-system, BlinkMacSystemFont, "Segoe UI", Roboto, Helvetica, Arial, sans-serif;
  background-color: var(--bg-color);
  color: var(--text-color);
  display: flex;
  justify-content: center;
}

#app {
  width: 100%;
  max-width: 600px;
  padding: 20px;
  box-sizing: border-box;
}

#board {
  display: grid;
  grid-template-columns: repeat(6, 1fr);
  grid-template-rows: repeat(6, 1fr);
  width: 100%;
  aspect-ratio: 1 / 1;
  background-color: var(--board-bg);
  border: 3px solid var(--border-bold);
  box-sizing: border-box;
  margin-bottom: 20px;
}

.cell {
  border: 1px solid var(--border-light);
  display: flex;
  align-items: center;
  justify-content: center;
  font-size: 2rem;
  font-weight: bold;
  user-select: none;
  cursor: pointer;
  box-sizing: border-box;
}

/* 6x6のブロック境界（2行×3列のブロック） */
/* 2行ごとの太線 */
.cell:nth-child(n+7):nth-child(-n+12),
.cell:nth-child(n+19):nth-child(-n+24) {
  /* This logic is slightly off, blocks are 2 rows x 3 cols? 
     Wait, 6x6 sudoku has blocks of 2x3. So rows are 1-2, 3-4, 5-6.
     We need bold borders after row 2 (cells 7-12) and row 4 (cells 19-24). */
  border-bottom: 3px solid var(--border-bold);
}
/* 3列ごとの太線 */
.cell:nth-child(6n+3) {
  border-right: 3px solid var(--border-bold);
}

.cell.hint {
  color: var(--text-color);
  background-color: #f9f9f9;
  cursor: default;
}

.cell.input {
  color: var(--primary-color);
}

.cell.selected {
  background-color: rgba(0, 122, 255, 0.1);
}

#pad {
  display: grid;
  grid-template-columns: repeat(3, 1fr);
  gap: 10px;
  margin-bottom: 20px;
}

.pad-btn {
  background-color: #fff;
  border: 1px solid var(--border-light);
  border-radius: 8px;
  padding: 15px 0;
  font-size: 1.5rem;
  font-weight: bold;
  text-align: center;
  cursor: pointer;
  user-select: none;
}
.pad-btn:active {
  background-color: #e5e5ea;
}

.action-btn {
  width: 100%;
  padding: 15px;
  border: none;
  border-radius: 8px;
  font-size: 1.2rem;
  font-weight: bold;
  margin-bottom: 10px;
  cursor: pointer;
}

#done {
  background-color: var(--primary-color);
  color: #fff;
}
#done:active {
  background-color: #0062cc;
}

#reveal {
  background-color: transparent;
  color: var(--primary-color);
}

.hidden {
  display: none !important;
}

#message {
  text-align: center;
  font-size: 1.2rem;
  margin-top: 20px;
}